Electrophotographic image formation generally consists of charging a surface of an electrophotographic photoreceptor, imagewise exposing the photoreceptor to light to form an electrostatic latent image, visualizing the latent image with a toner (development), transferring the toner image to transfer paper, and fixing the toner image by heat or pressure. This process using transfer paper (ordinary paper) is called indirect electrophotography and adopted in general electrophotographic copying machines using ordinary paper as a transfer material. There is another image formation process, called direct electrophotography, in which paper itself serves as a photoreceptor. In direct electrophotography, a latent image is formed directly on paper having a photosensitive layer (ZnO-coated paper), the latent image is developed with a toner, and the toner image is fixed on the paper.
Where an electrophotographic copying machine using ordinary paper as a transfer material according to indirect electrophotography is used for formation of a transparency on a transparent film (electrophotographic transfer film) in place of ordinary paper, various problems arise: for example, miss-feeding of films (e.g., feed of two or more films from a paper feeder at a time, hereinafter referred to as double feeding), haze, abrasion in handling, insufficient adhesion of toner, and emboss marks by a heating roll.
In general, fixing of a toner image on a transfer material inclusive of a transparent film is carried out by heat fixing by means of a heat roll or pressure fixing by means of a roller. Pressure fixing is attracting attention with expectation of size reduction of a copying machine and energy saving but has not yet come into general use on account of the failure in reaching the levels of fixing properties attained by heat fixing.
Heat fixing, which has conventionally been used, still involves problems waiting for solutions when applied to a toner image on a transparent film. For example, toner fixing properties on a transparent image-receiving layer, in terms of transfer and adhesion, are unsatisfactory as compared with those obtained by fixing on paper. Additionally, the image-receiving layer tends to be softened or melted by the heat of fixing to suffer uneven emboss marks.
The fixing properties might be improved by reducing the glass transition temperature (Tg) of the binder to be used in the image-receiving layer. However, mere use of a binder having a lower Tg would make the image-receiving layer more liable to softening and melting by the heat for fixing, resulting in development of uneven emboss marks.
In order to improve resistance to embossing and feeding properties of a transparent film, it has been a practice to incorporate a matting agent into an image-receiving layer. For example, an image-receiving layer comprising an acrylic resin, a styrene-acrylate copolymer or a polyester resin as a binder resin having dispersed therein a matting agent, e.g., silica powder, has been proposed as disclosed in JP-A-50-81339 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-1-315768 and JP-B-57-53592 (the term "JP-B" as used herein means an "examined published Japanese patent application"). These binder resins disclosed generally have a Tg of 60.degree. C. or higher. However, such image-receiving layers are not recognized to have sufficient toner fixing properties in terms of toner adhesion.
In an attempt to improve fixing properties (toner adhesion) and resistance to embossing, JP-A-61-259261 proposes to provide on a transparent substrate a coating layer comprising a potassium alkyl phosphate composed of ROPO.sub.3 K.sub.2 and (RO).sub.2 PO.sub.2 K (wherein R is an alkyl group having from 6 to 14 carbon atoms) and a cellulose resin; and JP-A-48-75240 teaches to combine a polymer and a matting agent with an antistatic agent, such as a naphthalenesulfonate, a sulfocarboxylic acid ester or a phosphoric ester. While incorporation of such an anionic surface active agent into an image-receiving layer brings about improvements in fixing properties, transfer properties and resistance to embossing, the surface active agent bleeds on the surface of the image-receiving layer with time, failing to maintain satisfactory toner transfer properties and adhesion to a toner.
On the other hand, it is known that toner transfer properties and film feeding properties are improved by providing a layer having controlled surface resistivity on a transparent plastic film. For example, JP-B-51-34734 discloses a plastic film having thereon a layer of an organic solvent-soluble resin containing a matting agent (inorganic fine particles or plastic powder) so as to have a surface resistivity controlled at 1.times.10.sup.9 to 1.times.10.sup.15 .OMEGA.. This resin layer exhibits somewhat improved toner transfer properties under a normal humidity condition. However, the resistivity increases under a low humidity condition on account of the organic solvent-soluble resin, tending to cause double feeding and, on the contrary, decreases under a high humidity condition, resulting in reduction in toner transfer properties. That is, resistivity control cannot be done sufficiently in using an organic solvent-soluble resin and requires assistance of a surface active agent or any other means.
JP-B-59-42864 discloses a film comprising a substrate having provided thereon two layers: a subbing layer of an anionic or cationic conductive resin and an image-receiving layer of an acrylic resin in this order, and JP-A-62-238526 discloses a film comprising a substrate having thereon a subbing layer containing an organic salt for imparting conductivity and an image-receiving layer of polymethyl methacrylate. A subbing layer comprising an ion conducting substance as taught above is effective to reduce the surface resistivity of the film surface. However, the surface resistivity is dependent on the water absorption of the subbing layer and is liable to large variation with changes in humidity. In particular, the resistivity is reduced under a high humidity condition to cause reduction in toner transfer properties.
In the field of direct electrostatic recording consisting of charging a recording material by applying high voltage with a recording stylus, developing the electrostatic latent image with a toner, and fixing the toner image (direct electrophotography), various recording materials containing fine particles of a metal oxide have been proposed to date. For example, a recording material comprising a paper substrate having provided thereon a conductive layer comprising a binder having dispersed therein metal oxide fine particles and further provided thereon a layer of an organic solvent-soluble resin containing calcium carbonate particles has been proposed, as described in JP-A-51-25140, JP-B-58-27494, JP-B-58-28574, JP-A-55-9524, JP-A-55-33134, and JP-A-56-38052. Application of the same technique to a transparent plastic film substrate in place of the paper substrate is also known. For instance, JP-A-61-151542 shows an electrostatic recording material comprising a transparent plastic film having provided thereon a conductive layer comprising antimony-doped stannic oxide particles having an average particle size of not more than 0.1 .mu.m and a binder (a water-soluble resin or an emulsion) and further provided thereon a dielectric layer.
Further, JP-A-56-143443 teaches an electrophotographic film for direct electrophotography as above noted, in which a conductive subbing layer comprising stannic oxide particles having an average particle size of not more than 0.5 .mu.m and a binder (a water-soluble resin) provided on a plastic film substrate.
The above-mentioned conductive layer containing metal oxide fine particles copes with change of environmental humidity and is expected to be applicable to a film for indirect electrophotography. However, so far as the aforesaid layer structure is followed, it is necessary to form a conductive layer containing metal oxide fine particles on a substrate and then an image-receiving layer containing a matting agent thereon. That is, two layers should be provided on a transparent substrate similarly to the above-described layer structure comprising an ion-conducting subbing layer and an image-receiving layer, which is very disadvantageous for productivity.
Further, the idea of the above-illustrated electrostatic recording materials for direct electrophotography does not apply as such to electrophotographic films for indirect electrophotography because the former should have far lower surface resistivity than the latter so that a latent image may be formed directly on the recording material. Neither does apply the construction of the above-mentioned conductive layer for direct electrophotography. That is, the films for indirect electrophotography are required to have satisfactory receptivity of the toner from a photoreceptor, i.e., toner transfer properties. In addition, since a large number of copies are usually taken by indirect electrophotography because of ease of copying operation, the films therefor are required to have excellent feeding properties. Other various problems as above stated are also waiting for solutions as well.
Binder resins for electrophotographic transfer films suitable for heat fixing preferably have a Tg of not less than 60.degree. C. so as to have satisfactory fixing properties and resistance to embossing in heat fixing with a heat roll. A combined use of a matting agent is effective especially for improving resistance to embossing. Nevertheless, it turned out that a combination of such a binder resin and a matting agent does not suffice to satisfy the above-mentioned various requirements, such as fixing properties, particularly when used under widely varying environmental conditions or when stored or used for a long period of time.